1. Field of the Invention
The present invention relates to the detection of chemical reaction components and, more particularly, to the detection of components introduced into a reaction zone in order to signal the beginning of a chemical reaction. The invention is particularly useful in the field of non-isotopic immunoassay for detecting the components of antigen-antibody reactions.
2. Description of the Prior Art
A common method for assaying antigens and antibodies is based on the fact that antigens react with their corresponding antibodies to produce a precipitate. The quantity of precipitate produced in an antigen-antibody reaction is proportional to either the antibody concentration or the antigen concentration, depending on which is present in excess. That is, for excessive antigen, the quantity of precipitate is proportional to the antibody concentration while, for excessive antibody, the quantity of precipitate is proportional to the antigen concentration. The quantity of precipitate produced is commonly determined with nephelometric techniques by measuring the extent to which the precipitate scatters a beam of light directed at the zone of the antigen-antibody reaction. However, a given quantity of precipitate may correspond to two possible values of antigen concentration depending on whether the antibody or the antigen is present in excess.
U.S. patent application Ser. No. 692,089, filed concurrently herewith, in the name of Robert J. Anderson et al. and entitled A System For Specific Serum Protein Analysis, discloses a system for the analysis of antigens and antibodies which measures the maximum value of the rate of change of the scattered light signal generated as the precipitate is formed during an antigen-antibody reaction. Significantly, as set forth in detail in the application, the time at which such maximum rate of change occurs following initiation of the antigen-antibody reaction has been found to indicate which of the two reaction components is present in excess. In other words, antigen excess is distinguished from antibody excess by observing the time after the start of the reaction at which the maximum rate of change of the scattered light signal occurs. Knowing which reaction component is present in excess enables the measured quantity of precipitate to be correlated to the correct one of the two possible values of antigen concentration. In order to measure the elapsed time between the start of the reaction and the maximum rate of change of the scattered light signal, it is necessary to first establish the reaction starting time. In this manner a timing clock can be started simultaneously with the reaction and used to measure the elapsed time to the maximum rate of change value.
Beyond the assay of antigens and antibodies, there are additional areas of chemical analysis for which it is necessary to ascertain the starting time of a chemical reaction. For example, in blood plasma prothrombin time determinations, a blood plasma sample and a clotting reagent are combined, and a measurement is made of the time required for the plasma sample to coagulate (clot). See for example U.S. Pat. Nos. 3,450,501 (Oberhardt) and 3,593,568 (Schmitz). Typically, the clotting is measured by a photodetector which detects light scattered by the clot as the clot forms and generates an electrical signal having a value which indicates the extent of clot formation. Obviously, in order to accurately measure the elapsed clotting time, it is first necessary to establish the starting time of the clotting reaction.
In the analysis of antigen-antibody reactions, as described in the aforementioned copending patent application, an operator manually pipettes the antigen and antibody reaction components, one at a time, into a reaction cell. Manual pipetting is also employed in prothrombin time determinations in the aforementioned Oberhardt and Schmitz patents. Oberhardt teaches that a timer may be started at the beginning of the reaction by means of a switch activated by depressing the pipette plunger. In this approach, a mechanical switch is attached to the pipette plunger, and as an operator depresses the plunger to eject the reaction component from the pipette, the switch is closed and completes a trigger circuit for signalling the start of the reaction. While manual switch triggering is satisfactory for some purposes, it is subject to a high degree of operator error. For example, if for some reason the pipette is empty, triggering will still take place when the pipette plunger is depressed even though nothing is actually ejected from the pipette. Moreover, if an incorrect reaction component is picked up in the pipette, triggering will still take place when this component is ejected into the reaction zone. Beyond this, it is possible to accidentally trip the switch and thus actuate the trigger circuit during an incorrect part of the analysis cycle. In order to prevent such accidental triggering, it is necessary to incorporate an "arming" switch on the analyzer control panel which prevents triggering unless the arming switch is actuated. However, an arming switch increases the operating and mechanical complexity of the system. Moreover, if the switch is left unarmed when the sample is introduced, the trigger will not function and the assay must be repeated.
A second approach for signalling the start of a chemical reaction is also suggested for the blood clotting reactions in the aforementioned Oberhardt and Schmitz patents. In the determination of blood clotting times, a detector monitors the scattering of light by the clot and generates a signal having a value indicating the extent of clot formation. In the two mentioned patents, when a reaction component is ejected into the reaction zone, a small upset or variation in the scattered light signal is detected to signal the start of the reaction. Unfortunately, this approach is often difficult to implement. First, the small variation in the scattered light signal is not a consistent and repeatable phenomenon. More importantly, and particularly as regards the nephelometric assay of antigen-antibody reactions, the scattered light signal ideally should exhibit no perceptible change at the time the last reaction component is introduced. This is because, with proper preparation, the antigen and antibody reaction components are essentially transparent and, thus, produce little if any light scattering. Thus it is evident that the scattered light signal does not provide an accurate measure of the start of the antigen-antibody reaction since the signal will not change perceptibly until some time interval after the start of the reaction when the precipitate begins to form.
From the above, it is evident that a need exists for a simple and dependable method and apparatus for monitoring components of a chemical reaction and for signalling the start of the reaction. The present invention fulfills these needs.